1. Field of the Invention
This invention relates generally to semiconductor manufacture, more particularly to alignment of semiconductor wafer-printing machines and more particularly still, to wafer-holding techniques, including an apparatus and method for holding semiconductor wafers at a uniform focus point for printing of a mask pattern upon the surface of semiconductor wafers.
2. Discussion of the Prior Art
Semiconductors are formed from semiconductive materials such as silicon and the like. Usually an artificial crystal of silicon such a P-type silicon to which a particular dopant substance such as boron has been added before crystallization from a hot melt, is sliced into a hundred or more slices or so-called wafers which can then be formed into individual semiconductor chips by a photographic curing process. Each slice or wafer of semiconductor material may be from about three to five inches in diameter, depending on the particular process and apparatus, and about twenty-five mils, or twenty-five thousandths of an inch, thick. The slices, or wafers, are first ground flat and polished to a mirror finish. Such slices, or wafers, are then heated in an oven containing oxygen and steam, causing a very thin layer of silicon oxide to form over the whole surface of the wafer where subsequent circuitry is to be formed. The silicon oxide film may be about 0.04 to 0.8 mils thick. A thin film of light-sensitive liquid plastic material referred to as "photoresist material" or "photoresist" is then applied to the surface of each wafer and this liquid is dried to form a solid photoresist film over the surface.
The coated wafer is then exposed to ultraviolet light, except in numerous, usually thousands, of tiny spots and strips where so-called N-regions are to be formed in the final semiconductor chips. To form the tiny spots and strips where ultraviolet light is not impinged upon the surface, the light is passed through a glass microfilm plate called a "photomask" that covers or shadows the wafer. Such photomask incorporates a photographically reduced pattern of very accurately spaced opaque or dark areas which shade the desired parts of the wafer or more particularly, the photoresist layer on the surface of the wafer from the light. As a result of such shading, the ultraviolet light only strikes a portion of the surface of the wafer where it cures the photoresist material, making such photoresist very strong and resistant to removal. The soft, unhardened areas of photoresist which have not been cured by the light because shadowed by the photomask are next washed away from the surface of the wafer with a solvent, leaving the hardened areas attached to the surface. Thereafter the wafer is dipped in an acid bath or otherwise treated with acid which etches or dissolves the oxide film in those portions of the surface where it is unprotected by the hardened or cured photoresist plastic layer. A type of acid is selected that does not affect the photoresist material or the silicon material, but only removes the silicon oxide film, resulting in an orifice or opening through the oxide layer which may be of the order of one mil in diameter.
The remaining hardened or cured photoresist material is next removed by a stronger solvent than was applied before, and the slice of wafer is placed in a very hot oven held at about 1,200.degree. C., where the slice is exposed to a gas containing a further dopant substance such as, in many cases, phosphorus, which is used to form N-type silicon. The phosphorus material in the gas diffuses into the silicon in the area of each hole through the oxide film, changing the underlying silicon from a P-type to an N-type silicon. The remaining oxide layer continues, however, to block diffusion to all other areas. Normally, the phosphorus may be diffused to a depth of about 0.08 mils or the like, after which the wafer is removed from the diffusion oven. The N-type regions are then recoated with an oxide layer by a second oxidizing treatment which forms a silicon oxide insulation area. In more complicated semiconductors, there may be several stages of diffusion forming various layers formed of various doped material.
As a final step in the production of the electrical circuit portion of the wafer assembly, a further set of holes is made through the oxide layer where electrical connections are desired and the slice or wafer is coated with a thin film of aluminum over the silicon oxide layer with the aluminum directly contacting the silicon through the holes in the silicon oxide. A new layer of photoresist material is then applied to the entire surface and the wafer is again exposed to ultraviolet light through a mask to map out a desired pattern of electrical conductive areas which will serve as circuit "wires" for connection of the various doped portions of the underlying silicon. After the photoresist material is then cured by exposure to ultraviolet light projected through the mask, the uncured portions are removed by a suitable solvent, after which the underlying aluminum may be etched away by an acid etch, leaving the aluminum electrical conductor portions protected by the cured photoresist material. Such cured photoresist may then be removed by a second solvent, or in some cases, may be left on the surface as an insulating material. As a final step, a number of individual semiconductor chips are carefully sawed by means of diamond blades out of the wafer severing repeating patterns of individual semiconductor chips formed in the material of the wafer.
As may be recognized, the exposure of a semiconductor wafer to ultraviolet light to cure the photoresist material, referred to generally as printing the wafer, constitutes a critical procedure in the production of individual semiconductor chips from a wafer. Since the circuit portions formed on the surface of the wafer, both in doping the surface of the wafer and forming electrical contacts on the surface of the wafer, are very small, it is important that the various areas be both placed and dimensioned extremely accurately so that effective and accurate circuits may be formed. This is accomplished by using a very accurately dimensioned mask with very sharp divisions between the transparent and the opaque portions of the mask and by having the ultraviolet light focused very accurately as it both passes through the mask and reaches the surface of the wafer. In order to obtain such accurate focusing, it is necessary to have the photomask accurately positioned with respect to the light source and to have the surface of the wafer accurately positioned with respect both to the light source and the mask.
It is fairly easy to accurately position the photomask from the light source accurately, but considerable difficulty has been encountered in the past in accurately positioning the wafer surface from the light source and mask.
In most installations, the wafer has been supported by a so-called stage plate, or xy base plane or focal plane plate, which holds the wafer. In one type of printing apparatus, such stage plate has been very accurately mounted at a predetermined distance from the light source. The stage plate is provided with wafer surface contact pads, or focal reference pads, against which the wafer is pressed during operation by a chuck which usually picks up the wafer by means of vacuum applied to the face of the chuck from a wafer transfer arm, after which the chuck moves into close proximity to the stage plate such that the surface of the wafer is positioned against the positioning pads, or focal reference pads, of the stage plate. After positioning of the wafer, the ultraviolet light source is activated or a shutter opened and the light is impinged on the surface of the wafer which, due to the original accurate positioning of the stage plate and the light source, is very accurately positioned on the wafer surface. Means are provided for checking the positioning periodically through a special tooling mask called a "focus wedge mask" which can check the distances very accurately, and means are provided for recalibration of the entire device. However, if a satisfactory production rate is to be attained or maintained, the alignment of the apparatus cannot be recalibrated between each application of a wafer to the stage plate, and anything which would tend to cause miscalibration can interfere with the accuracy of the circuits ultimately produced in the semiconductor chips.
A later type of automatic printer provides for automatic adjustment of either the light source or the stage plate, or both, after each wafer is positioned upon the stage plate so that the apparatus is continuously recalibrated. Such apparatus is considerably more expensive than the earlier or alternative apparatus having a fixed focus or focal length between the light source and the stage plate. In addition, the refocusing operation between the positioning of the wafer on the stage plate and the activation of the ultraviolet light takes a finite amount of time and therefore tends to slow down production of finished wafers.
One of the principal difficulties or drawbacks of fixed-focus machines has been that portions of the photoresist layer have tended to rub off the wafers so that over a period, a significant deposit of photoresist may build up upon the pads of the stage plate. Such deposit of photoresist material tends to hold or space subsequent wafers at increasing distances from the positioning pads so that the surface of the wafer from one wafer to the next tends to be displaced progressively farther from the light source. This effect has been counteracted in fixed-focus machines in the past largely by manually wiping the surfaces of the pads between every twenty-five to fifty, or even fewer, wafers to keep such pads clean and remove any buildup of material. Such manual wiping inherently takes a significant time and also requires opening up a normally closed apparatus exposing the wafer material to possible contamination and the like. Furthermore, the wiping procedure is not always successful and may result in a differential amount of photoresist material remaining on the various pads causing the wafer not only to be displaced away from the light source, but to be cocked slightly to one side, causing accentuated difficulties in forming accurate images upon the surface of the semiconductor material. Even with the development of an automatic wiping device which may operate during the period when the chuck is moving a new wafer into position, additional complications and variations are added.
Of course, the difficulty with photoresist buildup upon the focal reference pads, or positioning pads, may be substantially counteracted, but not alleviated, by the more costly and complicated automatic focus machines, but such machines have their own drawbacks, including not only greater initial cost, but more complexity with the result there are more things to go wrong with resultant additional downtime for repairs and greater operating cost as well as first cost.
There has been a need, therefore, for some method or means to prevent buildup of deposits of photoresist material upon the positioning pads of a stage plate in a fixed-focus wafer photo-printing apparatus.